The measurement of the distance between two objects has wide application in fields such as radar systems, indoor/outdoor positioning systems, robot systems, collision avoidance, game playing, and smart home/office arrangements. The distance measurement may be made by measuring the time-of-flight of wave propagation in air (or in another media) such as electromagnetic, light or sound waves. Multiplying the time-of-flight by the speed of the wave propagation enables the distance to be determined. The accuracy of such a distance measurement is therefore dependent on the measurement accuracy of the time-of-arrival (hereinafter referred to as TOA) or the time-difference-of-arrival (TDOA) of the incoming signal.
In a modern conventional ranging system, the incoming signal waveform is normally first sampled in the time domain before the TOA is measured. Although the true TOA is a continuous value, the TOA in a discrete system may only conventionally be estimated based on an integral number of sampling intervals ts. If the incoming time of a signal falls between two successive samples, in a simple ranging system the signal may be estimated as either coming at a first sampling time or a second sampling time, or in the middle of the two sampling times. Therefore, the final range estimation error is determined by the size of the sampling step. The ranging accuracy may be improved by increasing the sampling rate (that is, by shortening the sampling step). However, this increases the hardware complexity, as well as the cost and power consumption of the system.
The range between two objects may be measured in many different ways. Conventionally, determining the TOA of electromagnetic, sound and optical waves propagating in air is a common way to measure the distance between objects. As mentioned above, the accuracy of the TOA measurement will determine the accuracy of the final range (distance) measurement. For example, in an RF range measurement system, if the TOA error is 1 ns, the corresponding range error will be 0.30 m.
In many systems, to measure the range accurately, the arrival time of the leading edge of an incoming signal waveform needs to be accurately determined. In a discrete sampling based system, the estimation error of the leading edge is strongly dependent on the sampling step, that is, the time between samples. If the sampling step is reduced, the error will be reduced accordingly. The publication Time Domain Corporation, “PulsON 210™ Reference Design Training”, September, 2005 discloses an ultra wideband (UWB) ranging system based on such a method. However, in the described system, a high sampling rate is used which requires an accurate timing system and very high speed circuit implementation.
To detect accurately the leading edge of a waveform, a method is proposed in U.S. Pat. No. 5,977,958 which suggests the use of a high speed multibit analogue-to-digital (AD) sampling method. The slope of the leading edge of a waveform may be obtained from multiple samples of the incoming waveform and the exact threshold-passing time may be calculated. The TOA estimation may be obtained in sub-sampling step accuracy. However, this method requires the speed of the analogue-to-digital converter to be high enough for several samples to be taken in the signal's leading edge and, at the same time, the analogue-to-digital converter requires multibit resolution.
U.S. Pat. No. 6,587,187 B2 discloses a way of using multiple clocks to sample a waveform and find the leading edge thereof. A coarse clock and a fine clock are described. However, the timing system in this method is quite complex.
The above-mentioned conventional methods appear to require complex circuit implementation and the systems to work at a high clock rate. This results in high costs and high power consumption.
Thus there is a need for a system and method which is simple to implement and low in cost and power consumption.